This invention relates to the base designs of pressurized free-standing plastic containers. The base consists of three or more outwardly convexed legs to support the bottle standing upright.
A bottle generally consists of a shoulder portion with an opening, a sidewall or main body which is generally cylindrical in shape, and a bottom joining the sidewall. Because of the tendency of the wall of the pressurized plastic bottle to creep under internal pressure, it is a general practice to orient the plastic material and to design the bottle shape in such a way to improve resistance against creep. For the bottom section, it is known that if a uniform material distribution is achieved, the stress caused by the internal pressure can be minimized by using an outwardly hemispherical configuration. This bottom configuration, however, requires a separate means of support to make the bottle stand upright. For this purpose, a base cup is normally used.
Molecular orientation can be obtained by blowing a properly designed parison in a blow mold in the orientation temperature region. Normally, when a parison is blown in a bottle mold, the bottom portion near its center is somewhat thicker. To make full use of the material in the bottom area, the bottom shape can be made elliptical rather than spherical. Further, and importantly, such an elliptical configuration tends to make the tensile stress distribution in the bottom region more uniform.
One simple way to make a free-standing bottle is to use a champagne push-up bottom to creat a seating ring. One type of container incorporating such a construction is shown in U.S. Pat. No. 3,881,621. The difficulty with this method is that the bending force caused by the internal pressure tends to deform the structure and roll out the push-up portion near the seating ring. This results in a decrease of the size of seating ring reducing the bottle stability and also resulting in an increase of the length of the bottom. This type of deformation could eventually result in rocker bottom.
Another way to provide a self-support for the bottle is to incorporate into the bottom, three or more outwardly convexed legs disposed around the center pole of the bottom. Typical designs that have been proposed for such construction are shown in U.S. Pat. Nos. 3,598,270, 3,871,541 and 3,935,955. One objective of this invention is to provide such a design to provide a free-standing base for an oriented pressurized plastic container.
When a free-standing bottle with such a base structure is dropped from a certain height, the leg which hits the floor first tends to crack due to the impact. It is known that increased degree of orientation in the leg tip region tends to increase resistance against such an impact. Another objective of this invention is to incorporate enough orientation in the tip region of the legs to improve the impact strength of the bottom.
This type of free-standing base may still develop rocker bottom under internal pressure and high temperature. To increase the safety margin against such a rocker bottom problem, the clearance between the center of the bottom and the horizontal plane on which the legs stand should be large. In accordance with the invention, this is achieved by using an elliptical bottom to shorten the length of the bottom on one hand and by incorporating a small outwardly concave region at the center of the bottom on the other.
Since an outwardly concaved structure is less stable than an outwardly convexed structure, it is desirable to design a free-standing base with minimum portion of the outwardly concaved structure. This is achieved in this invention by using a relatively large portion of the elliptical bottom and by reducing the area of the transition region joining the main elliptical portion and the legs.